The present invention relates to an additional feature for use in a measuring sensor for the spectroscopic analysis of mediums, said measuring sensor comprising: a sample chamber for a gas mixture to be analyzed, which chamber is provided with flow connections for the flow-through of the gas mixture as well as with at least two windows transmissive to applied radiation, the inside surfaces of said windows defining therebetween a measuring absorption length; a radiation source for emitting radiation which progresses through the first window, the absorption length of the sample chamber, and further through the second window; one or a plurality of detectors which is or are optically directed towards this second window; at least one filter permeable to a certain wavelength band, or a radiation-dispersion producing means between the radiation source and each detector; as well as between the inside surface of the first sample chamber window and the radiation source a first length and/or between the inside surface of the second window and the detector or, respectively, detectors a second length, said lengths being in total at least about 20% of said absorption length. The invention relates also to a similar additional feature in a measuring sensor employing two or more filter-detector pairs. The invention relates also to a method for manufacturing such an additional feature for a light guide, comprising a housing which is made of a rigid material and includes radiation-transmitting ends and inner volume as well as a radiation-restricting inner surface and has a length which is at least about one and a half times over its mean diameter.
The above type of measuring sensors for the spectroscopic analysis of mediums have been disclosed e.g. in patents FI-95322, U.S. Pat. No. 5,254,858, U.S. Pat. No. 3,745,349, U.S. Pat. No. 4,468,561 and in the publication HEWLETT-PACKARD JOURNAL: R. J. Solomon--"A Reliable, Accurate CO.sub.2 Analyzer for Medical Use", September 1981. FIG. 6 in the present application illustrates a basic principle for such a device based on infrared absorption and used for analyzing gas mixtures, corresponding primarily to the design described in the publication FI-95322. This analyzer consists of the following basic components: a radiation source 1, a possible mirror 10, and a sample chamber 2 as well as one or a plurality of infrared detectors 9 and, between these detectors and the radiation source, in this case an optical bandpass filter 8. This makes for a non-dispersive measuring system for measuring a radiation R having passed through the sample chamber 2 for an absorption caused by a given gas component. In the case of FIG. 6, the spacing between the sample chamber 2 and the filter 8 and the detector 9 comprises a wave tube A, just like in FI patent 95322, said tube having a length L3 and producing a roughly parallel beam of rays R to the filter 8, in this case an interference filter. It is also prior known to employ various optical components, such as lenses, concentrators, and the like, between the radiation source 1 and the sample chamber 2 and/or between the sample chamber 2 and the filter-detector assembly 8, 9. The non-dispersive measuring system obtained by means of a filter or filters can be replaced with a lattice or grid distributing the radiation into various wavelengths for providing a dispersive measuring system. The non-dispersive measuring system can also be provided, not only with a plurality of parallel-connected filter-detector assemblies, but also with a single detector and replaceable filters. However, a drawback in all these above-cited publications is that at some point between the radiation source 1 and the sample chamber 2 and/or between the sample chamber 2 and the detector 9 there is a substantial space which is more or less freely accessible for the external atmosphere. As a result of this, the composition of the external atmosphere can have an effect on the measuring result. This could even be a major effect, particularly when measuring some gas component which is also present in the ambient atmosphere. Thus, in the prior art technology depicted e.g. in FIG. 6, a wave tube A includes an interior B which, due to its length, may cause a substantial absorption in a radiation R traveling therethrough. This problem is indeed recognized in the patent FI-95322 and, in order to correct it, it has been proposed that the wave tube interior B be filled with vacuum or with a gas that does not absorb on the wavelengths used in measuring. Another proposed alternative is that the wave tube interior be provided with a suitable absorbent, such as silica gel, zeolite, activated carbon, or calcium hydroxide. However, such a solution requires that the wave tube in question be sealed absolutely hermetically, which in practice has proved highly difficult in terms of achieving an absolutely reliable result. The amount and, if necessary, also the replacement of absorbent also makes for a more complicated structure and operation. This problem, i.e. faulty absorption occurring in the path of radiation away from the sample chamber, is not mentioned at all in the other above-cited publications. However, this possibility of faulty absorption exists in all constructions set forth in the cited publications. Furthermore, in actual real-life measuring equipment, the path of radiation includes several locations, besides the above-mentioned optional wave tube, in which this faulty absorption may occur.
The above-described faulty absorption caused by the ambient atmosphere may occur over any such section of an optical path extending from a radiation source to a detector, which is exposed to this external gas. Thus, in the prior art shown in FIG. 6, faulty absorption may occur over a length L3 between the sample chamber window facing away from the radiation source and the detector, in this case over a length L2a between the non-encapsulated radiation source and the sample chamber window facing the same, as well as over a double length L2b extending from the radiation source via a mirror 10 back to the sample chamber window.
The patent application FI-942547 describes a measuring sensor consisting of a measuring rod, wherein some of the radiation path is designed by using e.g. a sapphire rod with a totally reflective outer surface or a bundle of fibers, as long as it comprises a tube permeable to reflecting inwards the applied wavelength. However, neither does this publication pay any attention to the above-described possibility of faulty absorption since the publication mentions, as an equal alternative to the above rod or fiberoptics, also an internally reflective tube exactly like in FIG. 6 depicting the prior art. Indeed, the only purpose of such rod-shaped, tubular, or fiberoptics is, as explained in the cited publication, to enable the accommodation of a measuring gap in a large-diameter channel inside the same through an opening in the wall as well as to enable the measuring gap surfaces to remain warm and, thus, to prevent the moisture possibly present in the large flow channel from condensing on the measuring gap surfaces. The construction described in the publication is only good for measuring the concentration of a single gas component, since the channel can only be fitted with a measuring rod having a very small cross-sectional area. Since, in the cited construction, the measuring gap has a very small maximum length, this results in a factor limiting the selection of the absorption length of measuring. It should also be noted that, if, as suggested in this publication, the structure comprises a bundle of fibers which in a known manner retains the unchanged distribution of radiation having passed therethrough but, in addition to this, it is desirable to employ some other optical components for deflecting the radiation or for eliminating possible inhomogeneities, it will be necessary to use components external of the bundle of fibers. These external components and the distance required thereby again result in lengths or sections which are exposed to the external atmosphere and, hence, to eventual faulty absorption.
As for the above-mentioned faulty absorption, the following more detailed comments can be made. Firstly, a measuring error caused by faulty absorption is the larger the stronger is the absorption occurring in the free sections of a radiation path, i.e. within sections free or open to the ambient atmosphere, in relation to the absorption occurring in a gas mixture contained in the sample chamber. Such faulty absorption occurs e.g. whenever the free or open sections of a radiation path are subjected to disturbance gases which absorb radiation on the measuring wavelengths of an analyzer. According to the Lambert-Beer law, the strength of faulty absorption depends on the concentration of a disturbance gas and the length of the open sections of a radiation path. Fluctuation in the concentration of disturbance gases produces a varying-size, unknown measuring error, which therefore cannot be simply compensated for. In addition, the size or magnitude of an error depends not only on the concentration of a disturbance gas component but also on the temperature and pressure of disturbance gases, even if other participating factors should remain unchanged. As an example, it may be noted that carbon dioxide and water vapour contained in air can in many instances lead to significant measuring errors. On the basis of prior known technology, the effect of faulty absorption occurring over the exposed sections of a radiation path can be reduced by any of the following means. First of all, the analyzer can be provided with two-ray optics, wherein the infrared radiation is guided to a detector alternately through a sample chamber and a reference gas chamber and wherein the concentration is measured on the basis of the relationship between sample and reference signals. This type of structure is disclosed e.g. in the above-cited publication HEWLETT-PACKARD JOURNAL, September 1981. However, this necessitates the use of moving mechanical parts, which increases the size and price of an analyzer and decreases its reliability. Also, the sensor itself becomes bulky and sensitive to vibration and other outside influences. A second option is to remove the infrared-radiation absorbing disturbance gases from inside the analyzer and to seal the entire apparatus hermetically. This is practically inconvenient, restricts the number of mechanical structural alternatives for an analyzer, and increases its price. Furthermore, as pointed out above in terms of minor components, the hermetic sealing is problematic and, thus, the sealing of an entire analyzer is not generally possible. A third option is that the analyzer be sealed almost hermetically and disturbance-gases binding materials be introduced therein. Even such near-hermetic sealing is inconvenient to implement and increases the price of an analyzer. In order to afford a suficiently long service life for the disturbance-gases binding materials, such materials must be allocated a major space within the analyzer, which leads to a large-sized analyzer and, moreover, the operator must take care of the fixed-term replacements of these materials. A fourth option is to reduce the length of the free sections of a radiation path in the direction of a measuring beam with respect to the length of a sample chamber. However, this will restrict the structural alternatives of an analyzer and e.g. the number of gases that can be analyzed. In many cases, the number of gases to be analyzed is in practice limited to one and, in theory, perhaps to two, unless the efforts of developing electronic components lead to future radiation sources and detectors that are considerably smaller than before. A fifth option is to introduce into the interior of an analyzer a protective gas not containing disturbance gases. However, this requires a protective-gas supply assembly and possibly also a replaceable protective-gas container and, thus, the apparatus will become very bulky and inconvenient to use.
The distribution of radiation to a number of detectors can be effected in a prior known fashion by using fiberoptics, as disclosed in the publication U.S. Pat. No. 5,254,858, whereby a section of the free length of a radiation path may be composed of these optical fibers. Nevertheless, even this embodiment requires, at least between a sample chamber and fiberoptics, a concentrator or the like component and possibly a respective light guide between a radiation source and the sample chamber in order to produce as homogeneous a radiation distribution as possible. A similar distribution of radiation to a variety of detectors is implemented in patent FI-95322 by using a branched wave tube structure. In both these cases, however, the path of radiation used for measuring develops substantial free sections exposed to disturbance gases, especially concentrators in the former publication and especially wave tubes in the latter publication.